Question: Omar is 4 years younger than Christopher. For the last two years, Christopher and Omar have been going to the same school. Three years ago, Christopher was 3 times as old as Omar. How old is Christopher now?
Explanation: We can use the given information to write down two equations that describe the ages of Christopher and Omar. Let Christopher's current age be $c$ and Omar's current age be $o$ The information in the first sentence can be expressed in the following equation: $c = o + 4$ Three years ago, Christopher was $c - 3$ years old, and Omar was $o - 3$ years old. The information in the second sentence can be expressed in the following equation: $c - 3 = 3(o - 3)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $c$ , it might be easiest to solve our first equation for $o$ and substitute it into our second equation. Solving our first equation for $o$ , we get: $o = c - 4$ . Substituting this into our second equation, we get the equation: $c - 3 = 3($ $(c - 4)$ $ -$ $ 3)$ which combines the information about $c$ from both of our original equations. Simplifying the right side of this equation, we get: $c - 3 = 3c - 21$ Solving for $c$ , we get: $2 c = 18$ $c = 9$.